My quick 2 cents:
The goal of a parallel startup is to plunge the user into the graphical login as quickly as possible while the remaining daemons continue starting in the background. This gives the feel of a faster startup and in reality does just that with limited functionality as remaining services come to life.
Keep in mind, that for a login, you need network services in some environments (NIS, kerberos, ...) So the dependencies have to check for these needs also.
